In practice the decrease in flight time when going from 4 cell to 5 cell is 4 to 5 percent. A non-issue unless you're trying to get that very last flight out of a pack.
Errr not sure about that. I based my statement on looking at the charge figure on my Prodigy charger. I moved a couple of models, one a jet the other a 120 3D job, to 5 cell packs and noted that I was getting significantly higher figures. I fly 2 flights with the jet and 4 flights with the 3D then charge. I was definitely getting in the region of 15 to 20% more needed to recharge the 5 cell packs.
Nothing very scientific but it was noticeable that instead of 6 -700 mah I was getting 800 plus almost all the time. Both packs were Overlander 5 cell Nimh AA size with a nominal capacity of 1500 mah